While I know they are probably adequate, I hate the fact that the mini's (at least mine) does not have vented rotors on the rears.

Are there any vented rotors out there to replace the non-vented stock ones without materially effecting bracking bias?

 

in order to have vented rotors, they will need to be thicker, your stock calipers are not large enough to accomodate a larger rotor cross section.

the rear brakes on these cars are more than adequate, no one i know of has overheated / badly warped their rear rotors, they are fine as engineered by the good people at mini.